It would be lethargic, not playing, not running around a lot. Being lazy. Not eating his food and not drinking his water. Just not having any energy. You can usually tell by their eyes also.
These signs would be the same for all dogs - not just a yellow Lab Lab specifically. Watch to make sure that your dog is eating and drinking as he usually has been. Rejecting food is a telltale sign that something is amiss. Lethargy and lameness are other signs that there is a problem.
Signs of illness include increased sleeping, a disinterest in food, a disinterest in toys or activities that normally excite, discharge from the nose or eyes, and repeated vomiting or diarrhea.
Some signs of illness would be lethargy, loss of appetite, warm nose. Your dog could have eaten something that is bad for him and is vomiting or has diarrhea.
You know your dog best, so anything you perceive as unusual is cause for alarm. However typical signs of illness are: vomiting, diarrhea, lethargy, not eating/drinking, excessive sleeping..